A typical mass flow controller (MFC) is a device that sets, measures, and controls the flow of a fluid (e.g., a gas or a liquid). An important part of an MFC is a sensor that measures the mass flow rate of a fluid flowing through the device. The MFC compares an output signal of the sensor with a predetermined set point and adjusts a control valve to maintain the mass flow rate of the gas at the predetermined set point.
The mass flow sensor of an MFC is typically calibrated against a precision mass flow meter so that the output signal of the MFC sensor is adjusted (using calibration data) to match the measured flow of the precision mass flow meter. The calibration of mass flow controllers is typically performed by MFC manufacturers with a calibration gas, typically nitrogen (N2). Often, the obtained calibration data is gas-dependent—especially in the context of thermal mass flow sensors. As a consequence, when the MFC is operating with a gas other than the calibration gas, the calibration data may result in the MFC providing a flow rate that does not match the desired set point.
Due to the tendency of mass flow controllers to be inaccurate when the gas that is being controlled varies from the calibration gas, on-tool flow verification systems, such as the system depicted in FIG. 7, have been utilized to provide a reference flow measurement that is compared against the measurement of the MFC. These flow verification systems may utilize periodic pressure-based measurements of flow that are compared to the measurements of the MFC. For example, periodic rate-of-rise or rate-of-decay measurements are known to be utilized to determine whether the measured flow of an MFC has departed from the actual flow by virtue of variations in the composition of the fluid being controlled.
But these rate-of-rise and rate-of-decay systems merely provide reference information (e.g., as an alarm), and they may interfere with the flow of the fluid being controlled. As shown in FIG. 7 for example, a depicted rate of decay system is disposed in the same flow path as a mass flow controller; thus, interfering with the flow of the controlled fluid. Moreover, if the flow rate of the fluid being controlled is very high (e.g., exceeding 100 liters per minute), a very large containment chamber would be required for the rate-of-decay system of FIG. 7 to measure flow accurately. Thus, rate-of-rise and rate-of-decay systems are not typically used in connection with high flow rate systems.